PRAGMA secure_delete needed to actually delete deleted data from the storage

[brody4hire]

While checking https://www.w3.org/TR/webdatabase/ for some other areas of potential noncompliance I discovered W3 webdatabase DRAFT 7.2: Sensitivity of data:

User agents should treat persistently stored data as potentially sensitive; it's quite possible for e-mails, calendar appointments, health records, or other confidential documents to be stored in this mechanism.

To this end, user agents should ensure that when deleting data, it is promptly deleted from the underlying storage.

The plugin will not actually overwrite the deleted content unless the secure_delete PRAGMA is used.

SQLite keeps this setting disabled by default to improve performance.

The minimum solution is to document this limitation.

I think the correct solution would be for the plugin to add an option for this, and use this PRAGMA unless it is explicitly disabled by the option.